Numerous variations of opposed face-type seals, often referred to as mechanical seals, have been developed for use in creating a sealed relationship between a rotatable shaft and a surrounding housing. Such seals, including those of the type which are often referred to as gas seals, are conventionally positioned within a stuffing box or seal chamber which is defined within the housing in surrounding relationship to the shaft. In many types of equipment requiring use of such seals, however, and particularly pumps, the stuffing box or seal chamber is of an extremely small size (that is, the stuffing box is of a small radial clearance as measured between the inner diameter of the surrounding housing and the outer diameter of the rotatable shaft). For example, large numbers of pumps often referred to as ANSI pumps provide only about 5/16 to 3/4 inch radial bore clearance for accommodating the seal or packing, and positioning an effective opposed face-type seal in such small space has generally been extremely difficult, and hence other types of sealing or packing arrangements have often been utilized on pumps having small packing or stuffing box chambers (such pumps often being referred to as having small bore seal chambers). The problem is further complicated by the fact that such pumps, adjacent the end of the stuffing box, also generally provide little available space for mounting a seal exteriorly of the stuffing box.
While numerous mechanical seals of the opposed face type have been developed and utilized on fluid handling equipment including pumps, nevertheless most such seals have been undesirably large, both radially and axially, and hence have not been suitable for adaptation to and use on small bore pumps. For example, numerous double mechanical seals have been developed, including not only contacting face-type, but also gas seals which include both contacting and non-contacting face types. These known seals, however, in addition to exhibiting the conventional largeness which restricts their application to small-bore pumps, have also conventionally possessed the recognized problems of secondary seal hang-up and incompatibility of O-rings with the pumped fluid. In an effort to eliminate or minimize these latter problems, many of the known mechanical seals of both the gas and non-gas types have employed metal bellows to eliminate the O-rings as secondary seals and thus attempt to minimize the secondary seal hang-up and seal ring compatibility problems. The seals employing metal bellows, however, have generally employed rather large complex bellows arrangements which have typically employed multiple bellows convolutions provided with retaining flanges or elements at opposite ends, which retaining flanges in turn are either pressed, fitted or otherwise fixedly secured to the adjacent members of the seal assembly. These bellows have hence significantly increased the structural and manufacturing complexity of the seal assembly, and have resulted in the seal assembly being of significant size, particularly with respect to axial length. Such seal bellows also make it difficult to achieve a predictable and maintainable balance diameter when the bellows is subjected to the pressures of the pump process fluid on one side and a barrier fluid on the other side, and hence this inability to maintain an accurate balance diameter can cause variations in the balance pressure and accordingly can effect the amount of fluid which escapes or passes between the opposed seal faces, particularly in a non-contacting-type gas seal.
The use of conventional bellows in known seals, as discussed above, further complicates the proper application of forces to the seal rings, particularly with respect to the application of these forces relative to the centroids of the seal rings, and thus maintaining proper convergence and hence proper opposed contacting or adjacent relationship between the opposed seal faces is further complicated. The fact that many of the known bellows also have a collar at one end which typically has a shrink fit onto the seal ring also induces additional stresses into the seal ring which can interfere with proper seal ring configuration and hence can affect the proper convergence of the seal faces.
Accordingly, it is an object of this invention to provide an improved face-type double seal, particularly a gas seal, having a desirably small and compact configuration so as to be particularly adaptable for use on pumps having small bore seal chambers, which improved seal is believed to overcome many of the disadvantages associated with conventional known seals, as briefly summarized above.
More specifically, this invention relates to an improved gas seal having inboard and outboard seal units each defined by a pair of opposed and relatively rotatable seal faces, one face of each pair having appropriate grooves which communicate on one seal periphery with a pressurized barrier gas, preferably an inert gas such as nitrogen. The inboard seal unit, at its other seal periphery, communicates with the process fluid being handled by the rotating equipment, namely the pump. Each seal unit has one of the seal rings, the nonrotating ring in the preferred embodiment, urged axially toward its opposed ring by a compact bellows which axially cooperates between the respective seal ring and a backing member. The bellows is positioned to create an isolation between the barrier and process fluids and to provide a mean effective diameter to provide desirable balance of pressures as imposed on the axially moving seal ring to optimize seal performance.
In the improved seal of this invention, as aforesaid, the bellows is preferably of a single convolution defined primarily by two bellows plates or leaflets which are sealingly engaged along one radial edge, the inner radial edge in the preferred embodiment, and have their other radial edges (the outer radial edges in the preferred embodiment) respectively sealingly engaged against the axially movable seal ring and the backing member. The bellows is preferably free of retaining collars and the like so as to simplify the bellows construction and size, and one of the bellows plates is preferably maintained in a nonfixed but contacting sealing engagement with an opposed face formed on the adjacent seal ring. This greatly simplifies the overall compactness as well as manufacture and assembly of the seal, and minimizes the application of fixation forces on the seal ring which tend to create undesirable distortion thereof. The nonfixed seal contact point or points with the bellows are preferably provided with a gasket-type material associated therewith, such as by coating the exterior surface of the bellows with a suitable gasket-like material, or by providing a gasket between the bellows and the opposed contact surface, which gasket can be either fixed to and captivated on the bellows or mounted on the opposed contact surface.
The improved seal of this invention, due to the structure and functional cooperations created by the bellows, minimizes or eliminates the secondary seal drag problem created when the secondary seal is defined by conventional O-rings, the bellows is still able to accurately maintain a proper pressure balance diameter without experiencing the significant variations which have been experienced with conventional elongate multiple convolution bellows as used in prior seals, the bellows enables the seal to operate with higher temperature process fluids without encountering problems of seal degradation or incompatibility with the process fluid, the bellows can be constructed free of end collars or the like and hence can be engaged with the adjacent seal ring and backing member solely through a sealed pressure contact to simplify the construction and minimize undesired distortion forces, particularly on the seal ring. The overall seal can be manufactured in a very small and compact size so as to be particularly adaptable for mounting exteriorly of but directly adjacent the stuffing box of a pump, with the overall seal being effectively constructed preferably as a cartridge to facilitate its adaptation to the shaft of the pump.
The improved seal, as aforesaid, also includes a balance piston or pusher associated with the inboard seal unit so that, upon failure of the barrier pressure, the process fluid pressure will effectively shift the pressure piston to effect a sealed closure with the respective seal ring, preferably on the barrier fluid side of the bellows, to prevent escape of process fluid.
The improved seal, as aforesaid, also prevents the application of significant unbalanced process fluid pressures on the seal rotor.
Other objects and purposes of the invention will be apparent to persons familiar with structures of this general type upon reading the following specification and inspecting the accompanying drawings.